Capacitive measurement and/or detection systems have a wide range of applications, and are among others widely used for the detection of the presence and/or the position of a conductive body in the vicinity of an electrode of the system. A capacitive sensor, called by some electric field sensor or proximity sensor, designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which could comprise the one or more antenna electrodes themselves—at which the influence of an object or living being on the electric field is detected.
The different capacitive sensing mechanisms are for instance explained in the technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith et al., published in IEEE Comput. Graph. Appl., 18(3):54-60, 1998. The paper describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three-dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode is measured, whereby the displacement current may be modified by the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
The capacitive coupling is generally determined by applying an alternating voltage signal to a capacitive antenna electrode and by measuring the current flowing from said antenna electrode either towards ground (in the loading mode) or into the second electrode (receiving electrode) in case of the coupling mode. This current is often measured by means of a transimpedance amplifier, which is connected to the sensing electrode and which converts a current flowing into said sensing electrode into a voltage, which is proportional to the current flowing into the antenna electrode.
Some capacitive sensors are designed as sense-only capacitive sensors having a single sense electrode. Also, quite often capacitive sensors are used that comprise a sense electrode and a guard electrode that are proximally arranged and mutually insulated from each other. This technique of “guarding” is well known in the art and is frequently used for intentionally masking, and thus shaping, a sensitivity regime of a capacitive sensor. To this end, the guard electrode is kept at the same electric AC potential as the sense electrode. As a result, a space between the sense electrode and the guard electrode is free of an electric field, and the guard-sense capacitive sensor is insensitive in a direction between the sense electrode and the guard electrode.
For example, patent document U.S. Pat. No. 8,354,936 B2 describes a capacitive passenger detector for a vehicle. The capacitive passenger detector includes a main electrode, a sub-electrode and a guard electrode. The main electrode and the sub-electrode are separated apart from each other, and disposed in a seat of a vehicle. The guard electrode is disposed between the main electrode and a body of the vehicle, and separated apart from the main electrode. A sensitive characteristic measurement unit is configured for applying an alternating voltage signal to the main electrode, the sub-electrode and the guard electrode selectively or totally and for converting a current generated in the main electrode, the sub-electrode and the guard electrode to a voltage, respectively. The capacitive passenger detector further comprises a controller that defines a current flowing through the guard electrode to be a reference current when a voltage of the main electrode and a voltage of the guard electrode have the same potential. The controller defines a current flowing direction of the current flowing through the guard electrode to be a negative direction when the voltage of the main electrode is higher than the voltage of the guard electrode. The controller defines the current flowing direction of the current flowing through the guard electrode to be a positive direction when the voltage of the main electrode is lower than the voltage of the guard electrode. The controller corrects the voltage of the main electrode based on the current flowing through the guard electrode so that a corrected voltage of the main electrode is set to be a passenger determination data. Even when a potential difference is generated between the main electrode and the guard electrode, the controller detects the capacitance of the passenger correctly.
Capacitive sensing systems which are used in the control of airbag systems or other safety-related applications may be considered as safety-relevant system components. It may thus be necessary to monitor the good functioning of the different components of the sensor (sensing electrode and/or guard electrode) in order to rule out a false reading by the capacitive occupancy or proximity detection system.
With the currently available systems, complexity and costs that are required for detecting a sensor interruption, in particular a sense electrode interruption or a guard electrode interruption, are quite high. Current solutions include means such as but not limited to a complex interruption measurement circuit indeed, a diode between the sensing electrode and the guard electrode, a four-wire cable for foil sensor, four crimps, four connector pins, etc.